The present invention relates to an improved process for the preparation of toluidines by the reductive deamination of 2,3-diaminotoluene, 3,4-diaminotoluene, or mixtures thereof.
2,3- and 3,4-diaminotoluene are constituents of toluylenediamine, which is produced on a large scale. In the conventional further processing of toluylenediamine to toluylene diisocyanate, 2,3- and 3,4-diaminotoluene, which are collectively referred to hereafter as o-diaminotoluene, would interfere and are therefore separated off beforehand, generally by distillation. This o-diaminotoluene which has been separated off is used for the preparation of various chemical products. However, the amount of o-diaminotoluene obtained is frequently greater than that which is required for these applications.
The object of the present invention is, therefore, to utilize excess o-diaminotoluene in an economically and ecologically sensible manner.
It is known that o-diaminotoluene can be catalytically hydrogenated to form toluidines. This hydrogenation is accompanied by the elimination and release of ammonia and is therefore referred to as reductive deamination. Since both ammonia and, particularly, toluidines are valuable key chemicals, the reductive deamination of o-diaminotoluene is a very sensible reaction in terms of the object of the present invention.
U.S. Pat. No. 3,532,754 describes a process for the preparation of o- and m-toluidine by the reductive deamination of o-diaminotoluene. That process comprises reacting o-diaminotoluene with hydrogen under elevated pressure at temperatures of 200.degree. to 250.degree. C., in the presence of supported catalysts based on cobalt oxide. However, this process suffers from serious disadvantages.
This process produces inadequate yields of toluidine. It is evident from Examples 1 and 4 of the '754 patent that the total yield of toluidine after a reaction time of 4.5 or 7 hours at a reaction temperature of 225.degree. C. is only 52 or 46%, respectively, of theory, based on the starting o-diaminotoluene. This is partly due to the fact that the conversion of the starting o-diaminotoluene is low. For example, the conversion in Example 1 of the patent after a reaction time of 4.5 hours at 225.degree. C. is only 64%.
For a reaction time of 5.25 hours at 225.degree. C., Example 3 of the patent presents numerical results which suggest that the conversion of the starting o-diaminotoluene is less than 20%. However, as a reaction temperature of 225.degree. C. is exactly within the range 215.degree. to 235.degree. C., which is disclosed therein as the preferred range, the cobalt catalysts described in the patent have to be regarded as unsuitable for this reaction since they only produce inadequate conversions of the starting o-diaminotoluene.
On the other hand, U.S. Pat. No. 3,532,754 (at column 2, lines 23 to 29) discloses that the reductive deamination of o-diaminotoluene at temperatures of about 250.degree. C., in the presence of the cobalt catalysts, results in the formation of toluene, i.e., reductive deamination and hence decomposition of the desired reaction products, and that, at even higher reaction temperatures, ring hydrogenations also take place to form methylcyclohexylamine and methylcyclohexane. This statement is further confirmed by the examples of this patent.
Similar to toluene, methylcyclohexylamine and methylcyclohexane are comparatively worthless products, and must be removed by costly procedures. The formation of these by-products in the reductive deamination should be suppressed as far as possible. According to the '754 patent, this is achieved by choosing appropriately low reaction temperatures. As explained above, however, uneconomically low conversions have to be accepted as a consequence of these low reaction temperatures.
A further disadvantage of said process is that it yields practically no p-toluidine, if any. As shown by Example 1 and claims 7 and 8 of the '754 patent, the reaction of o-diaminotoluene by the described process results only in o-toluidine and m-toluidine, with only trace quantities of p-toluidine being detectable in the reaction mixtures. From a statistical viewpoint, this circumstance is surprising. It suggests that the described cobalt catalysts have a selective action in the reaction, and thereby prevent the formation of p-toluidine. However, it is precisely p-toluidine which is a valuable key chemical. Thus, the probable selectivity of the cobalt catalysts in the reductive deamination of o-diaminotoluene results in a considerable disadvantage.
Finally, the process of the '754 patent also suffers from the disadvantage that the reactions demand high concentrations of cobalt catalysts. According to the patent, the preferred concentrations are 30 to 60 parts by weight of catalyst, based on 100 parts by weight of starting o-diaminotoluene. These high concentrations inevitably result in a marked deterioration in the space-time yields. In particular, they require considerable technical effort, particularly in the working-up of the reaction mixtures, for example, in the separation of the catalysts by filtration or in the separation of the mixtures by distillation.
U.S. Pat. No. 4,329,501 describes another process for the preparation of toluidines by the reaction of 2,3- and 3,4-diaminotoluene with hydrogen under elevated pressure. This process is characterized in that cobalt metal is used as the catalyst, and the reactions are carried out in the presence of water as solvent.
In the '501 patent, the described experimental examples are intended to verify the mode of action of the claimed process. The results of these examples are collated in a table. However, it is unclear which of the described examples are to be regarded as being representative of the process described by the '501 patent, and which are comparative examples. In particular, the results of the described experiments are only given in the form of relative contents, determined by gas chromatography (area %), of the reaction mixtures from which the catalyst has been removed by filtration, without indicating their absolute amounts or, where appropriate, their water content. Also, since there is no data on the amounts of recovered starting material, this table gives no information either on the conversions of starting o-diaminotoluene achieved or on the yields of desired toluidine obtained. Thus, these examples are of little use in assessing the claimed process.
This table of the '501 patent does, however, show that the times of the described reactions are about 10 to 20 hours, i.e. relatively long. This is not surprising since o-diaminotoluene is used in solution in a solvent and, in accordance with the law of mass action, reacts more slowly than in undiluted form.
Therefore, the object of the present invention is to provide an improved process for the preparation of toluidines by the reductive deamination of 2,3-diaminotoluene and 3,4-diaminotoluene. This improved process should result in minimal quantities of by-products, and, simultaneously, high conversion rates of the starting diaminotoluene.